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Five myths commonly associated with neutering in dogs

02 November 2014
11 mins read
Volume 5 · Issue 9


There are some commonly held beliefs that do not accurately reflect the current understanding of how sex hormones affect health and behaviour in dogs. This article identifies five of these myths and indicates that: neutering is very unlikely to make dogs calmer; castration will not improve all problem behaviours in male dogs; pseudopregnancies can occur in spayed as well as entire bitches; delmadi-none (Tardak) is not a reliable indicator of the behavioural effects of castration; neutering can have negative as well as positive effects on the incidence of health problems in dogs.

Recent research suggests that between 54% and 75% of pet dogs in the UK are neutered (Diesel et al, 2010; PDSA, 2013), indicating that neutering is a very commonly performed procedure in general practice. However there are some myths associated with neutering that are fairly widely accepted by veterinary surgeons and veterinary nurses in practice as well as amongst pet owners but that do not necessarily reflect the current understanding of how sex hormones and neutering affect health and behaviour in dogs. Five of these are discussed below.

Myth 1: neutering makes dogs calmer

It is widely believed that neutering will help calm boisterous, over-excitable adolescent dogs down. However this is often not the case. In most cases these dogs are receiving insufficient mental stimulation and/or physical exercise for their age and breed and neutering will not alter this. Boisterous, adolescent dogs will benefit most from ensuring their needs for exercise and mental stimulation are met, including provision of suitable outlets for any breed specific behaviours they may have been selected to perform such as chasing, tracking or searching. Owners also often inadvertently encourage dogs to show over-excitable behaviour through failing to notice and reinforce calm behaviours (Figure 1).

Figure 1. Owners also often inadvertently encourage dogs to show over-excitable behaviour through failing to notice and reinforce calm behaviours.

Veterinary nurses should ideally be able to give appropriate advice to owners of over-excitable adolescent dogs, or, if there is no one with suitable expertise within the practice, be able to refer the owner to an appropriately qualified dog trainer or behaviourist because behaviour problems in adolescent dogs are a significant risk factor for them being rehomed or even put to sleep.

Myth 2: castration will improve all behaviour problems in male dogs

It seems to be widely assumed that castration will solve all behaviour problems in male dogs and this is often a first intervention in a dog showing problem behaviour, before any comprehensive behavioural assessment and advice are given. This can be problematic because although castration will improve some problem behaviours it is often more effective alongside behaviour modification, and in some dogs castration can potentially have adverse behavioural effects.

Unsurprisingly, castration is most likely to improve problem behaviours that are directly influenced by circulating testosterone, including roaming in search of in-season bitches, indoor urine marking, mounting (other dogs, people, inanimate objects) and confident, competitive aggression directed specifically at other male dogs (Hart and Eckstein, 1997). Circulating testosterone can also increase the likelihood of dogs being very distracted by other dogs or showing signs of high arousal and frustration, particularly around bitches in season. However for a number of reasons castration does not always reliably eliminate these problem behaviours (Hart and Eckstein, 1997). Masculinisation of the male brain, which occurs due to exposure to testosterone before birth, is not affected by castration. Also, some of these behaviours can have other causes (e.g. mounting can be a stress response or an attention-seeking behaviour) and learning can maintain behaviours after the original hormonal influences have been removed (Hart and Eckstein, 1997). From personal experience these behaviour problems are most likely to be improved successfully if castration is combined with behaviour modification to address any learned aspects and/or non-hormonal causes of the behaviour.

Compared with inter-male aggression, aggression in other situations is less directly influenced by circulating testosterone. However circulating testosterone can lower the threshold at which aggression is shown and increase its intensity through its effects on a number of other behavioural traits including increasing vigilance and responsiveness to threats, behavioural persistence and risk-taking behaviours, all of which can increase the likelihood of an animal responding to a threat with aggression (Eisenegger et al, 2011; Overall, 2013; Terburg and van Honk, 2013). Therefore castration can potentially reduce the likelihood of a male dog showing aggression in response to threats, although in animals that are already showing aggression this effect is only likely to be significant if combined with a comprehensive behaviour modification plan to address the dog's underlying emotional state and the triggers that cause him to show aggression.

Castration is very unlikely to have any effect on problem behaviours that do not differ markedly between dogs and bitches, for example those arising from boredom/under stimulation, unwanted attention-seeking behaviours or predatory behaviours such as inappropriate chase behaviour or hunting and killing wildlife. These will need to be addressed through identifying and addressing the underlying cause of the problem behaviour.

Although in many cases castration will either have a beneficial effect on behaviour or no effect at all, there are a couple of ways in which castration can potentially cause or exacerbate problem behaviours in dogs.

Increasing fearfulness

Circulating testosterone is associated with increased self confidence and reduced fearfulness (Terburg and van Honk, 2013) so castration can potentially increase fearfulness, especially in dogs that are already nervous. For dogs that are fearful of strangers, of being handled, or of being in unfamiliar places, the experience of being hospitalised and handled for the castration operation itself may be scary and further increase fearfulness in these situations.

If it is necessary to castrate a fearful dog, there are things that can be done to reduce the likelihood of the castration operation itself causing the dog to become more fearful, and this also applies to spaying fearful bitches. It is a good idea to do some behaviour modification beforehand to accustom the dog to being handled by unfamiliar people in the surgery environment and also to wearing a Baskerville muzzle so they can be handled safely with minimal restraint. Ideally there will be someone within the practice with the expertise to do this but if not it is wise to enlist the help of a suitably qualified behaviourist.

Careful management of fearful dogs on the day of the operation will also help, including: premedicating them as soon as they arrive at the surgery; allowing the owner to wait with their dog until they become drowsy; reducing stressors in the hospital/kennel environment such as noise and the view of other dogs; having bedding or other familiar items from home so the dog is surrounded by familiar scents; and keeping handling as calm and gentle as possible. If not contraindicated for other reasons, giving a benzo-diazepine such as midazolam or diazepam may be beneficial, as benzodiazepines can impair long-term memory formation (Beracochea, 2006).

Increased attractiveness to other male dogs

Occasionally after castration some dogs become highly attractive to other male dogs, who behave much as they would with an entire bitch: sniffing intrusively around their anogenital regions and making persistent attempts to mount them. This is likely to be associated with changes in pheromones produced after castration. Sometimes there is an obvious underlying cause such as an anal gland infection which can alter the properties of the pheromones produced (Mills et al, 2013), and once treated the problem may stop. A similar effect can be seen in dogs that have been bathed in shampoos containing compounds such as methyl parabens (Methyl p-hydroxybenzoate) (Person, 1985). Before castration these effects may have been masked by testosterone. However sometimes there is no obvious underlying cause that can be treated and dogs may remain attractive to other male dogs long term. This can be a significant problem because, unsurprisingly, dogs usually find these attentions from other dogs distressing and will often show defensive aggression in response.

Figure 2. Castration is most likely to improve problem behaviours that are directly influenced by circulating testosterone such as indoor urine marking.

We do not currently have enough information about the behavioural effects of neutering dogs before puberty compared to afterwards to know how this influences the incidence of problem behaviours in the longer term and more research is needed to clarify this. In theory pre-pubertal castration should reduce the incidence of problem behaviours in male dogs that are directly influenced by testosterone. However, it could potentially increase the likelihood of other problem behaviours. For example there are reports of pre-pubertally-neutered dogs of both sexes showing immature adolescent-typical behaviour, including lack of emotional control such as poor tolerance of frustration and over reactivity to stimuli, into adulthood. Studies in other species including humans indicate that during adolescence sex hormones play an important role in the development of normal adult social behaviour, including effects on cognition and emotional regulation (Schulz et al, 2009). Although more research is needed, it is possible that this also applies to dogs.

Myth 3: Tardak reliably indicates the behavioural effect of castration

It can sometimes be difficult to predict what effect castration might have on an individual dog's behaviour. For example, in a dog that is showing aggression because it is fearful, circulating testosterone can potentially increase the likelihood of the dog showing aggression in response to threats, but removing testosterone could further increase its fearfulness. In these dogs it can be useful to assess what effect reducing testosterone will have on behaviour before performing an irreversible surgical procedure.

Delmadinone (Tardak) has traditionally been used to reduce both testosterone and fertility in dogs. It is a progestogen that exerts negative feedback on the anterior pituitary to suppress release of both luteinising hormone (LH), important for testosterone production, and follicle stimulating hormone (FSH), important for production of spermatozoa. However, like most progestogens, Tardak also enhances the effect of gamma-aminobutyric acid (GABA), the main inhibitory neurotransmitter in the brain, resulting in a calming effect completely unrelated to its effect on testosterone. So while Tardak can be extremely useful for short-term suppression of testosterone-related behaviours, such as frustration in response to an in-season bitch, it is not necessarily a good indicator of the behavioural effect of castration.

The only agent currently available that can give a reversible and fairly accurate indication of the behavioural effect of castration is a deslorelin implant (Suprelorin, Virbac). Deslorelin is a gonadotrophic releasing hormone (GnRH) agonist and works by over stimulating the GnRH receptors in the anterior pituitary until they become desensitised and down-regulate, reducing production of LH and FSH. After implantation there is an initial increase in testosterone for approximately 2 weeks before it starts to fall, reaching post-castration levels about 4–6 weeks after implantation and remaining so for approximately 4–6 months (for the 4.7 mg implant), after which testosterone gradually increases again (Trigg et al, 2006; Gobello, 2007; Ramsey, 2011). During the period when testosterone is low the dog's behaviour should be fairly representative of their behaviour after castration (Steur, 2011), and if improved it would be sensible to arrange surgical castration before the implant wears off. If the dog's behaviour deteriorates significantly during this time it would probably be wise not to castrate them.

Because of Suprelorin's mode of action there can potentially be a temporary worsening of any testosterone-related behaviour in the 2 weeks after implantation and owners must be made aware of this. It is also important to be aware that Suprelorin implants are not licensed for use either to influence problematic testosterone behaviours in dogs or to assess the potential effectiveness of castration, so their use in any of these situations is off-label.

Myth 4: only entire bitches can have false pregnancies

Although we usually associate false or pseudopregnancies with entire bitches they can also occur in spayed bitches. If a bitch is spayed when she already has a pseudopregnancy the physical and particularly behavioural signs can persist indefinitely if they are not recognised and treated (Harvey et al, 1999). It is therefore very important that, before being booked in for spaying, bitches that have had at least one season are checked for any physical signs of pseudopregnancy including mammary enlargement and milk production and owners asked about the presence of any behavioural signs that could be consistent with a pseudopregnancy. These can include aggression, often in the context of guarding resources such as food, bed or other resting place or toys, nesting behaviour including digging, irritability and increased reactivity to stimuli or sometimes depression and reduced reactivity.

If a pseudopregnancy is suspected the bitch should be treated with the anti-prolactin drug cabergoline (Galastop, CEVA) until all physical and behavioural signs have gone, before she is spayed in order to prevent the pseudopregnancy persisting after spaying (Harvey et al, 1999). Although the Galastop data sheet indicates that a course of 4–6 days should be sufficient to treat signs of pseudopregnancy in bitches it often needs to be given for at least 2 weeks until the behavioural signs have completely gone (Ramsey, 2011).

Bitches can also develop a pseudopregnancy if they are spayed in the metoestrous period immediately following a season when progesterone is raised. Removing the ovaries leads to a sudden drop in progesterone which can trigger the release of prolactin from the anterior pituitary and precipitate a pseudopregnancy. This is most likely if a bitch is spayed in the 2 months after a season but post-spay pseudopregnancies have been reported in bitches spayed up to 5 months after a season (Harvey et al, 1999).

Key Points

  • Routine neutering is currently the most reliable means of population control in dogs, and for this reason dogs should be neutered unless there are strong behavioural or health-related reasons not to do so.
  • Neutering can have both beneficial and adverse effects on health and behaviour in dogs and bitches. While more research is needed into this, dispelling some of the more common myths associated with neutering, and particularly how it affects behaviour in dogs, should help us to make the most appropriate decisions for individual dogs and their owners.
  • In male dogs castration can be beneficial in reducing the risk of developing problem behaviours that are directly influenced by testosterone, but in dogs that are already showing testosterone-related behaviour problems, castration is likely to be more effective if done alongside a behaviour modification programme to address learned and any other non-hormonal aspects of the behaviour.
  • Care must be taken when neutering fearful dogs and bitches to reduce the chances that the experience of being hospitalised and handled by unfamiliar people will add to their fearfulness. Removing testosterone can further increase fearfulness in nervous male dogs.
  • Post-spay pseudopregnancy should be considered as a possible cause of sudden behaviour changes, including increased aggression, after bitches are spayed.
  • Behaviour changes usually become apparent shortly after spaying and mirror those seen in entire bitches. Although some of these bitches also have enlarged mammary glands and milk production these are not always present and if they are not there is a risk that the behaviour changes may not be recognised as a hormonally-related problem. Careful history taking may be needed to ascertain exactly when the problem behaviour started, particularly if the problem has been present for a while before behavioural help was sought.

    Pseudopregnancy in spayed bitches will usually respond to cabergoline (Harvey et al, 1999) although, as in entire bitches, a course of 2 weeks or longer may be needed to completely resolve the behavioural signs.

    Myth 5: the relationship between neutering and the incidence of health problems in dogs is clear and well understood

    It has long been recognised that neutering is associated with various health benefits including a reduced risk of mammary tumours and pyometra in bitches and of testicular cancer and prostatic hypertrophy in male dogs. Conversely it is also widely believed that spaying significantly increases the risk of urinary incontinence in bitches. However some recent studies suggest that the health-related risks and benefits of neutering are not necessarily as clear cut as previously thought.

    Although there does seem to be a link between spaying and urinary incontinence in bitches, a review of earlier studies suggests that this link may not be as strong as was previously thought (Beauvais et al, 2012). Studies also suggest that neutering can be associated with an increased risk of some health problems unrelated to the urogenital tract including obesity, joint problems, some types of cancer and age-related cognitive problems, and that these risks may be further influenced by whether dogs are neutered before or after puberty (Root Kustritz, 2007; Sanborn, 2007; Reichler, 2009; Torres de la Riva et al, 2013; Hart et al, 2014). Further research is needed to clarify these findings and to determine how reliably they can be generalised to all dogs. However it is important to be aware of the potential risks as well as benefits when routinely neutering dogs in practice.


    Because neutering is such a commonly performed procedure in general practice it is important that veterinary surgeons and veterinary nurses understand where neutering might have adverse as well as beneficial effects on health and behaviour in dogs, and that this understanding is based on up to date information and research rather than myths.

    Population control is one of the most important benefits of neutering, and until more reliable non-surgical means of contraception become available dogs should be neutered unless there are good health-related or behavioural reasons not to neuter. It is also important to prevent unwanted matings in any dogs that are left entire for non-breeding purposes.

    Being aware that castration and spaying can potentially have adverse as well as beneficial effects on behaviour in dogs should make it easier to avoid problems, either by not neutering particular animals (a consideration in fearful male dogs that are not showing aggression or any other problem behaviours likely to be directly related to testosterone) or in animals showing problem behaviours, to ensure neutering is supported by a behaviour modification programme from a suitably qualified behaviourist to address any other underlying causes of the problem behaviour that are not directly related to levels of sex hormones. Taking care over how animals are managed and handled at the time of neutering, particularly those that are already fearful, should reduce the chances of this procedure further increasing fearfulness. Being aware of the possibility of persistent pseudopregnancy as a cause of problem behaviour in spayed bitches will also increase the likelihood of this condition being identified and treated after spaying.

    Dispelling some of the common myths associated with neutering should help veterinary professionals make the most appropriate decisions for each individual dog and owner and also ensure that owners do not have unrealistic expectations regarding how neutering will influence the behaviour of their dogs.